In front of public buildings in Lower Saxony acc. to source usually three flags are hoisted. The images show the flags the heraldic way. It means, the flag displayed left in image from the point of view of the visitor is the right flag from point of view from inside the building. The flag on the (heraldic) right side is privileged. There are two sets of flags flown on flag days in Lower Saxony:
Klaus-Michael Schneider, 9 Nov 2016

Set one consists of Europe flag, German flag and flag of Lower Saxony.
It is hoisted on the following days:
27 January: Flag Days in Lower Saxony
Commemoration Day of Victims of National Socialism
9 May: Europe Day
Klaus-Michael Schneider, 9 Nov 2016

Set two consists German flag, flag of Lower Saxony and local flag.
It is hoisted on the following days:
1 May: Labour Day
23 May: Anniversary of Announcing the Constitution (Grundgesetz)
1 June: Anniversary of Legal Validity of Lower Saxonian Constitution
17 June: Commemoration Day of People's Insurrection against Communist Government in the DDR
20 July: Commemoration Day of Resistance against National Socialism
3 October: Day of German Unity (National Holiday)
2nd Sunday before 1st Advent (National Mourning Day)
days of public elections
Klaus-Michael Schneider, 9 Nov 2016